Yesterday, French gymnast Samir Ait Said suffered a horrible fracture (warning, graphic picture at the bottom of this post) of his tibia/fibula during the vaulting event. Gymnastics as a sport carries a high degree of risk, and to me, vaulting might be the most dangerous event. The gymnast runs as fast as possible, uses the vault to launch themselves as high as possible into the air, and then performs a series of flips and twists before landing. Here is the video of the injury if you want to watch it. Gymnasts, and athletes in other sports, such as basketball and volleyball, land repeatedly during practice and games/competitions, and these types of fractures are very rare. So, what went wrong to cause this injury and others similar to it (think about Kevin Ware in the NCAA basketball tournament a few years ago).
First, because Said projected himself so high in the air, when he landed he applied a large amount of force to the ground, and due to Newton's 3rd law of motion (for every action there is an equal and opposite reaction), the ground applied a large amount of force back to his foot. Also, because gymnasts are taught to "stick the landing" in order to maximize their score, a large amount of force is applied in a very short time period (the product of the average amount of force and the time over which it is applied is called impulse, which applies to almost every Olympic sport/event). Now, the mat helps dissipate some of this force, but it was still likely hundreds of pounds of force, probably 4-5 times greater than his own body weight. However, he has probably landed from this same height with around the same amount of force hundreds of times during his gymnastics career, and never suffered an injury like this. So, what really went wrong this time?
If you watch the video, you will notice that he did not complete his last rotation, and therefore was not able to get his feet directly underneath his body, or his base off support was not directly under his center of mass. Ideally, when landing from a jump, it's best if the feet are directly underneath the person' center of mass. Going back to Newton's third law, this will ensure that the ground reaction force is applied in the vertical direction and will be transferred directly up the person's lower extremity. When Said landed, his feet were not directly under his body, so instead of the force from the ground being applied mainly in the vertical direction, it is applied at about a 45 degree angle from the ground. The created a bending moment around the midpoint of his tibia and fibula, and because of the large amount of force, both the tibia and fibula broke. I'll do another post discussing the relationship between the amount of loading placed on a bone and how much it can deform before breaking. While this was a devastating injury for Said, hopefully with surgery and many months of rehabilitation he can make a full recovery. It also demonstrates that in many sports, the most dangerous part is landing from a jump. While fractures like this are pretty rare, other injuries such as ankle sprains, achilles strains/tears, and ACL tears often times occur when landing from a jump.
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